Cholera, an ancient disease, has shown a remarkable ability to persist and spread in the modern world, with an estimated disease burden of 1.3 to 4.0 million cases per year worldwide. Cholera struck Haiti in October, 2010, in the aftermath of the massive earthquake on January 12 of that year. These were the first cholera cases reported in Haiti in over 100 years; the clonal nature of isolates from early cases is consistent with a single-source introduction of the microorganism. A total of 754,735 cholera cases and 9,068 deaths have now been reported in Haiti, including over 30,000 cholera cases in 2015, reflecting significant ongoing issues with disease control, and underscoring the need for continued basic and applied cholera research. Cholera in Haiti represents a novel natural experiment, involving introduction on an island of a single clone of toxigenic Vibrio cholerae O1, at a known point in time. As such, it provides a unique opportunity to address key questions about the disease, with implications for our ability to understand and control cholera globally (with particular relevance for parts of Africa, which may more closely resemble Haiti than Bangladesh or India, where cholera studies have traditionally been conducted). Building on our existing Haitian research infrastructure, we propose to explore a series of questions: a) What are the relative contributions of human-to- human vs. environment-to-human transmission to occurrence of cholera in Haiti? b) What are the drivers for symptomatic infection among cholera patients? c) Will environmental reservoirs persist over time? and d) What are the drivers for observed evolutionary changes within V. cholerae O1 strain populations? Specific Aims to address these questions, with associated working hypotheses, are as follows: Specific Aim 1: Use of household studies to explore routes of cholera transmission, rates of asymptomatic illness, and drivers for clinical illness. Working hypotheses: a) In Haiti, environmental-to-human routes of transmission are responsible for more cholera infections than human-to-human transmission; and b) Occurrence of symptomatic illness is associated, at least in part, with specific patterns of intestinal bacterial microflora, and/or absence/reduced numbers of ICP2 and, possibly, other lytic cholera bacteriophage. Specific Aim 2: Monitoring of aquatic environmental reservoirs for V. cholerae O1. Working hypotheses: a) Aquatic environmental reservoirs for V. cholerae O1 will persist in Haiti for at least the next 5 years; and b) Frequency of environmental isolation of V. cholerae O1 lacking genes critical for human virulence will increase as environmental reservoirs ?age? across time. Specific Aim 3: Phylodynamic analysis of V. cholerae O1 strains. Working hypotheses: a) Environmental isolates represent independent, long-term reservoir populations of the microorganism, displaying a slower rate of evolution and a slower, albeit significant, population growth, and b) Both clinical and environmental strain populations undergo positive selection, with selection driven by presence of cholera-specific bacteriophage.